Use nullibility to infer optionality of service and body parameters

[halter73]

app.MapPost("/todos", (Todo? todo) => { 
// ...
app.MapGet("/something", (IService? service) => { 
// ...

In cases like the above, we could use the nullability of the Todo and IService parameters to determine weather to request body and service are optional.

See #31658 (comment)

Thanks for contacting us.

We're moving this issue to the Next sprint planning milestone for future evaluation / consideration. We would like to keep this around to collect more feedback, which can help us with prioritizing this work. We will re-evaluate this issue, during our next planning meeting(s).
If we later determine, that the issue has no community involvement, or it's very rare and low-impact issue, we will close it - so that the team can focus on more important and high impact issues.
To learn more about what to expect next and how this issue will be handled you can read more about our triage process here.

[davidfowl]

Also detect default values:

app.MapPost("/todos", (Todo todo = null) => 
{ 
});

Essentially signals from the type system that the user is willing to tolerate null.

[JamesNK]

Required reflection metadata about whether parameters have default values and are nullable is not available on private and compiler-generated methods.

Also semi-blocked on dotnet/runtime#29723 (having this API is nice, but we can do this ourselves if API is not available)

[davidfowl]

We want to determine if these parameters should fail to bind if there's no value provided for them from the determined source (inferred or explicit). The question is, how does the developer indicate that a parameter is optional (should allow default value) or not. For example

string Hello(string name = "John") => $"Hello {name}";
app.MapGet("/hi", Hello);

In the above, if no name is provided (via the query string), the value will be "John".

string Hello(string name = null) => $"Hello {name}";
app.MapGet("/hi", Hello);

In the above, if no name is provided, the value will be null.

string Hello(string name) => $"Hello {name}";
app.MapGet("/hi", Hello);

What does the above mean? Should it mean non-null? Today if no name is provided the value will be null.

We need a way to opt-into "required-ness":

• Nullable annotations:
• Validation Attributes
• Absence of default values could mean required
• Type system (e.g. F# option)

// Required name
string Hello(string name) => $"Hello {name}";

// Required name
string Hello([Required]string name) => $"Hello {name}";

// Optional name
string Hello(string name = null) => $"Hello {name}";

// Optional name
string Hello(string? name) => $"Hello {name}";

The same would apply for body parameters:

// Required TodoItem
async Task<IResult> PostTodo(TodoItem todo)
{
    await db.Todos.AddAsync(todo);
    await db.SaveChangesAsync();

    return Created($"/todo/{todo.Id}", todo);
}

// Required TodoItem
async Task<IResult> PostTodo([Required]TodoItem todo)
{
    await db.Todos.AddAsync(todo);
    await db.SaveChangesAsync();

    return Created($"/todo/{todo.Id}", todo);
}

// Optional TodoItem (can be null)
async Task<IResult> PostTodo(TodoItem todo = null)
{
    if (todo is null)
    {
       return UnprocessableEntity();
    }

    await db.Todos.AddAsync(todo);
    await db.SaveChangesAsync();

    return Created($"/todo/{todo.Id}", todo);
}

// Optional TodoItem (can be null)
async Task<IResult> PostTodo(TodoItem? todo)
{
    if (todo is null)
    {
       return UnprocessableEntity();
    }
    await db.Todos.AddAsync(todo);
    await db.SaveChangesAsync();

    return Created($"/todo/{todo.Id}", todo);
}